# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1444420703 -7200
# Node ID 92dade31295323c9972c62a1de810d7123730c74
# Parent  a02bebefc205122dccb09ba49cb427444e5ed9dc
Propagate errors to horizontally-adjacent pixels for nicer dithering.

diff -r a02bebefc205 -r 92dade312953 optimiser.py
--- a/optimiser.py	Fri Oct 09 19:01:18 2015 +0200
+++ b/optimiser.py	Fri Oct 09 21:58:23 2015 +0200
@@ -39,7 +39,7 @@
     return min(max(v, lower), upper)
 
 def clip(v):
-    return within(v, 0, 255)
+    return int(within(v, 0, 255))
 
 def distance(rgb1, rgb2):
     r1, g1, b1 = rgb1
@@ -392,9 +392,14 @@
                 value = get_value(rgb, most)
                 im.putpixel((x, y), value)
 
+                if x < width - 1:
+                    rgbn = im.getpixel((x+1, y))
+                    rgbn = tuple(map(lambda i: clip(i[0] + (i[1] - i[2]) / 4.0), zip(rgbn, rgb, value)))
+                    im.putpixel((x+1, y), rgbn)
+
                 if y < height - 1:
                     rgbn = im.getpixel((x, y+1))
-                    rgbn = tuple(map(lambda i: clip(i[0] + i[1] - i[2]), zip(rgbn, rgb, value)))
+                    rgbn = tuple(map(lambda i: clip(i[0] + (i[1] - i[2]) / 2.0), zip(rgbn, rgb, value)))
                     im.putpixel((x, y+1), rgbn)
 
         im.save(output_filename)